A strain wave gearing is configured from a rigid internally toothed gear, an externally toothed gear having flexibility (elastic deformability), and a wave generator, and is free of the backlash that occurs in normal gears and is able to achieve a large reduction ratio in one stage.
Known examples of typical strain wave gearings include the cup-shaped strain wave gearing comprising a cup-shaped externally toothed gear disclosed in Patent Document 1, the top-hat-shaped strain wave gearing comprising a top-hat-shaped externally toothed gear disclosed in Patent Document 2, and the flat strain wave gearing comprising a cylindrical externally toothed gear disclosed in Patent Document 3.
In a strain wave gearing, the externally toothed gear is made to flex into a non-circular shape, typically an ellipsoidal shape, by the wave generator, and is meshed with the internally toothed gear in two locations of the major-axis direction of the ellipsoidal shape. When the wave generator is rotated by a motor or the like, the meshing positions of the two gears move in the circumferential direction, and relative rotation occurs between the two gears, the rotation corresponding to the difference in the number of teeth of the two gears. Fixing one gear in place makes it possible to acquire reduced rotational output from the other gear. Patent Document 4 proposes a shape for a wave generator that causes an externally toothed gear to flex.
Another example of a strain wave gearing is one in which a flexible internally toothed gear is disposed on the external peripheral side of a rigid externally toothed gear, and a wave generator is disposed on the external peripheral side of the internally toothed gear.
Also known are the frictional engagement wave devices proposed in Patent Documents 5 and 6. A frictional engagement wave device comprises a rigid member having a circular frictional engagement internal peripheral surface, a flexible member having a circular frictional engagement external peripheral surface disposed on the inner side of the rigid member, and a wave generator disposed on the inner side of the flexible member. When the flexible member is made to flex into a non-circular shape, e.g., an ellipsoidal shape, by the wave generator, the circular frictional engagement external peripheral surface flexes into an ellipsoidal shape. This results in a state in which the portions equivalent to major-diameter positions in the frictional engagement external peripheral surface are in frictional engagement with the circular frictional engagement internal peripheral surface of the rigid member.
When the wave generator is rotated, the frictional engagement positions of the two members move in the circumferential direction. The peripheral length of the frictional engagement external peripheral surface is shorter than that of the frictional engagement internal peripheral surface by a predetermined amount. Consequently, when the wave generator makes one rotation, relative rotation proportional to the peripheral lengths occurs between the two members. Fixing one member in place so as to not rotate makes it possible to acquire reduced rotation from the other member.
Frictional engagement wave devices are also known to have configurations in which a flexible member is disposed on the outer side of a rigid member, and the flexible member is made to flex and mesh with the rigid member by a wave generator disposed on the outer side of the flexible member.